1. Field of the Invention
This invention relates to ionomer resins, to transparent ionomer resin films, and in particular relates to individual sheets of ionomer resin films and ionomer resin films for use in laminates, including laminated glass, and further relates to ionomer resins which are neutralized with polyamines to produce thicker, stronger transparent films.
2. Description of Prior Art
Safety glass can be reinforced by lamination with an inner layer of polycarbonate. The resulting lamination, however, is impractical for two principal reasons. One reason is insufficient bond strength when the polycarbonate is bonded directly to the glass. A second, and even more important reason stems from polycarbonate and glass having different co-efficients of thermal expansion. In safety glass laminates wherein polycarbonate is bonded directly to glass, the polycarbonate can crack and craze on cooling from the temperature necessary to bond the two together, because of the different thermal expansion co-efficients of the components.
Initial attempts to solve these problems involved interposing additional interlayers of polyvinyl butyral between the polycarbonate and the glass. Adhesion between the polycarbonate and the glass proved insufficient unless a plasticizer was also used. However, when a plasticizer was used, the plasticizer often caused the polycarbonate to develop stress cracks, and accordingly, to have low light transmission properties.
The initial problems appear to have been solved by using the laminated safety glass described in U.S. Pat. No. 3,888,032, which has achieved wide commercial success. The laminate comprises polycarbonate reinforced glass wherein the polycarbonate and glass are bonded to one another by an interlayer of polyurethane. Polyurethane provides sufficient adhesion to glass and to the polycarbonate, and no stress cracking or cloudiness develops in the product.
Despite the commercial success of polyurethane laminated product, there has been a continuing effort to develop less expensive products, particularly as polyurethane is an expensive component. This invention provides new glass laminates, with and without layers of polycarbonates, and other reinforcing transparent plastics, which are considerably less expensive than the polyurethane laminates, yet which at the same time are every bit as satisfactory, if not more so, with regard to adhesion, strength and clarity. Laminates according to this invention comprise at least one layer of glass laminated with an ionomer resin film.
In the specification and claims the term "ionomer" or "ionomer resin" mean an extrudable resin comprising ionically crosslinked ethylene-methacrylic acid and ethylene-acrylic acid copolymers. Properties which distinguish these ionomer resins from other polyolefin heat-seal polymers are high clarity, tear resistance, abrasion resistance, solid-state toughness and resistance to oil-fat permeation. The starting ionomer resins are generally partially neutralized with one or more cations. Preferably the cations are selected from the group consisting of alkali metals, aluminum, ammonium salt and zinc. Advantageously, the cation is sodium alone or together with zinc. However, as will be discussed hereafter, the esters or the non-neutralized form of the resin are also adaptable to the present invention.
Various grades of ionomer resins are available for extrusion coating and film extrusion. It is also known that ionomer resins can be co-extruded with other plastic resins and exhibit adhesion to other polyolefins, nylon resins and coextrudable adhesive resins often used as bonding layers in multi-ply coextruded structures. A very wide variety of partially neutralized ionomer resins are manufactured by E.I. DuPont de Nemours and Company under the registered trademark "SURLYN".
Ionomer resins have been suggested for use primarily in the area of packaging, for foods, liquids and pharmaceuticals, as well as certain industrial applications including lightweight sails, bonded cable sheath, roof underlayments and flame retardant products. In most applications, ionomer resins are offered as a superior substitute for polyethylene. Moreover, it has been generally assumed that ionomer resin films thicker than 10 mil cannot be obtained while still maintaining optical clarity of at least 60% light transmission.
Layers of ionomer resins can be formed by casting, forming blown film or extrusion, the latter being preferred. Once formed, there are no significant differences between cast, blown and extruded layers. When the ionomer resin layer is sufficiently thick, polycarbonate layers can be eliminated altogether in forming layered materials, and if the ionomer layer can be made sufficiently thick without interfering with optical clarity, an unsupported film can be provided.
Ionomer resins have several advantages over polyurethane. Polyurethane is difficult to manufacture, is expensive and is hard to fabricate. Also, polyurethane is frequently not clear enough for use in windshields and the like. By contrast, ionomer resin films can be easily extruded to desired thicknesses, and at about one-half the material cost of polyurethane. Ionomer resins have demonstrated better adhesion characteristics to glass and polycarbonates, as well as better resistance to lower temperatures. In preferred embodiments, the surface to which the ionomer resin is bonded may be primed to get good adhesion, as is the case with polyurethane. Silane coupling agents are suitable primers. With regard to optical properties, ionomer resins demonstrate better clarity than polyurethanes when prepared according to the invention.
When films of the ionomers of copolymers of ethylenemethacrylic acid or ethlyene-acrylic acid have previously been formed, they usually only retain their clarity when formed in very thin films. The clarity of the films is insured because the ionomers can be cooled quickly after being melted. Rapid cooling prevents finely dispersed crystalites from being formed and, thereby, creating a hazy film. These crystalites lower the light transmission of the film and give lower clarity to the film. In thicker films and sheets of the ionomer, the degree of clarity becomes an important problem since a larger mass of the film cools much more slowly and allows the crystalites a greater opportunity to form and grow. In fact, clear sheets of 20 mils or thicker are not obtained with clarity under normal cooling conditions. Rapid quenching of the thick layers can help, but rapid quenching becomes impossible or at least very difficult if the ionomer sheet is laminated or is a part of a larger object. When transparent windshields are to be made from ionomer films, some means is required in order to prevent the crystalites from forming and creating the resulting haze in the film during processing and cooling.
The present invention primarily concerns development of diamine neutralized carboxylic acid-containing hydrocarbon polymers which can be formed into transparent sheets or films which are substantially thicker than previously thought obtainable.
U.S. Pat. No. 3,471,460 to Rees also teaches diaminemodified acrylic or methacrylic acid hydrocarbon copolymers, and in the discussion thereof indicates that diamines may also be used as modifying or neutralizing materials. The present invention, however, provides a group of diamines which are an improvement over that patent in that only primary diamines and polyamines are preferred. The Rees patent includes many diamines, but does not include: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 are each alkyl groups of 1-4 carbon atoms, and M is CR, nitrogen, phenyl and cycloalkyl having 5-8 carbon atoms, wherein R is hydrogen, alkyl of 1-4 carbon atoms, lower alkyl amine, or cycloalkyl of 5 to 8 carbon atoms.
The present invention preferably excludes all diamines or polyamines that are not primary or aryl groups.
The present invention also excludes aromatic primary amines, i.e. ##STR2## and focuses on diamines and polyamines that have one or more (H.sub.2 N--CH.sub.2 --) groups per molecule.
The amine groups in --CH.sub.2 NH.sub.2 structure should form the strongest interaction with free carboxyl groups in the ethylene methacrylic acid or ethylene-acrylic acid copolymers or in the acrylic acid homopolymers. Therefore, the selected group of diamines which includes BAC, 1,6 hexane diamine and 1,12 dodecanediamine, form the strongest amine salt or ionomer bonds. These diamines form stable ionic bonds at the highest temperatures of all the amines, at which point these polymers will be in their most disordered and non-crystalline state. By forming strong amine salt bonds these diamines cross-link these polymers and fix them in their least crystalline form. Upon cooling these polymers become "frozen" in their non-crystalline form and remain optically clear. The broader general groups of amines that Rees described include mostly amines that will not produce significant optical clarity in the resulting neutralized polymer. The selected groups of diamines of the present invention form strong diamine ionomer bonds and the resulting polymers are tougher, stronger, and have greater optical clarity on the average than the groups of diamines which Rees teaches.